The present invention relates to a process for preparing bismaleimides, and more particularly, to a process for preparing aromatic ether bismaleimides. The invention also relates to an improved and more efficient process for preparing bismaleamic acids which are the precursors of the desired bismaleimides.
Currently, bismaleimide resins are being favorably considered as replacements for epoxy resins because of their greater thermal stability and lower moisture sensitivity. Current epoxy resins have a maximum useful temperature of 350.degree. F. and are weakened by absorbed moisture, particularly when used as matrix resins in fiber reinforced composites. However, the currently available commercial bismaleimides are brittle and are not competitive with epoxies in terms of toughness.
Aromatic ether bismaleimides promise to have greater toughness and lower moisture retention than other bismaleimides because of the ether groups in the main chain and the relatively large monomer chain length. Preliminary data indicate that this type of structure is tougher than state of the art bismaleimides and moisture retention levels are around 2% compared with 4-5% for conventional commercial bismaleimides.
As a result of the aforementioned properties, a great commercial interest in aromatic ether bismaleimides exists at this time. Aromatic ether bismaleimides find application as composite matrix resins for structural composites and electric circuit boards, and as adhesives for bonding structural materials. The aromatic ether bismaleimides may be used alone or in conjunction with other co-monomers.
U.S. Pat. No. 3,839,287 discloses typical polyarylimides which are prepared from aromatic diamines and maleic anhydride. The diamines are reacted with maleic anhydride and then the product is chemically converted to the imidized form by addition of acetic anhydride and triethylamine, or sodium acetate. These polyarylimides are taught to be useful in the preparation of glass cloth prepregs, molding materials, and adhesives.
U.S. Pat. No. 4,288,583 discloses a curable mixture which contains maleimides and phenols. The maleimides may be aliphatic or aromatic, and are prepared by reacting the appropriate amines or polyamines with the maleic anhydride in a polar solvent and in the presence of a catalyst.
U.S. Pat. No. 4,464,520 discloses aromatic bismaleimides which are prepared by using the usual imidization reaction where the imidization is carried out in an inert aprotic solvent using a slight excess of maleic anhydride. Useful inert aprotic solvents include dimethylformamide, dimethylsulfoxide, and dimethylacetamide. Typical reaction temperatures are 40.degree.-60.degree. C. with a preferred range of 50.degree.-60.degree. C. Typical reaction times are 1-1.5 hours. The resulting polybismaleimides are useful as binders in composite molded components, and for circuit board manufacture.
U.S. Pat. No. 4,460,783 is similar to U.S. Pat. No. 3,839,287 in disclosing first reacting aromatic ether diamines with maleic anhydride to form bismaleamic acids which then are reacted with acetic anhydride in the presence of a catalyst such as potassium acetate to convert the amic acid groups to imide groups.
Among aromatic ether bismaleimides, particular commercial interest exists in a bismaleimide of the Formula (I) ##STR2## which is derived from Bisphenol A. This bismaleimide is disclosed in U.S. Pat. No. 4,460,783 to Niahikawa et al and U.S. Pat. No. 3,839,287 to Kwiatkowski et al. This compound reportedly can be prepared by reacting Bisphenol A with 4-chloro-1-nitrobenzene to yield the compound 2,2-bis4(4-nitrophenoxy)phenyl)propane which is reduced to the corresponding diamine 2,2-bis[4-(4-aminophenoxy)phenyl]propane and reacted with maleic anhydride to produce the bismaleimide. The latter reaction proceeds in two stages, first, with formation of the maleamic acid derivative, and then with imidization. For the reasons discussed infra, it is believed that neither the Niahikawa nor the Kwiatkowski process produces the bismaleimide of Formula (I) in significant yield and free of large concentrations of co-products.
Several problems are encountered when the Niahikawa reaction sequence is used on an industrial scale. On an industrial scale, the aromatic diamine reactant is employed at a high concentration in the reaction solvent. As the conversion of the aromatic diamine to the maleamic acid increases, the maleamic acid precipitates from the reaction mixture and the reaction mixture becomes too thick to stir even at low concentrations. The mixture has the appearance of an aerosol foam. On a large scale, the temperature cannot be controlled and the mixture does not react uniformly. As a result, a product is obtained which contains high concentrations of by-products. This product is disadvantageous because these by-products interfere with the normal overall curing mechanism of the bismaleimide, and in addition, generate excessive volatiles during cure which results in voids in the cured products.